1. Field of the Invention
The present invention relates to a crystallization apparatus for irradiating an amorphous or polycrystalline semiconductor film with a laser beam to produce a crystallized semiconductor film, an optical member for use in the crystallization apparatus, a crystallization method, a thin film transistor, and a display apparatus. The present invention particularly relates to an apparatus and method in which an amorphous or polycrystalline semiconductor film is irradiated with a laser beam phase-modulated using a phase shift mask to produce a crystallized semiconductor film.
2. Description of the Related Art
A material of a thin film transistor (TFT) for use in a switching device for controlling a voltage to be applied to a pixel, for example, of a liquid crystal display (LCD) has heretofore roughly been classified into amorphous silicon, poly silicon and single crystal silicon.
Amorphous silicon can obtain a high withstand property. Poly silicon has an electron mobility higher than that of amorphous silicon. Therefore, a transistor formed by poly silicon has advantages that a switching speed is high, a response of a display is high, and a design margin of another component is reduced as compared with a transistor formed by amorphous silicon. In addition to a main body of a display, peripheral circuits such as a driver circuit and DAC can be incorporated in the display. In this case, these peripheral circuits can be operated at a higher speed.
Poly silicon is constituted of an aggregate of crystal grains, and has lower electron or hole mobility than single crystal silicon. Moreover, in the thin film transistor (FET) formed using the poly silicon, fluctuation of the number of crystal grain boundaries existing in a channel portion is a problem. To solve the problem, a crystallization method of producing poly silicon having a larger grain diameter has been recently proposed in order to enhance the mobility of electrons or holes and to reduce the fluctuation of the number of crystal grain boundaries in the channel portion of each FET.
As this type of crystallization method, a “phase control excimer laser annealing (ELA)” has heretofore been known in which a polycrystalline or amorphous semiconductor film is irradiated with an excimer laser beam via a phase shift mask to produce a crystallized semiconductor film. Details of the phase control ELA are described in, for example, “Surface Science Vol. 21, No. 5, pp. 278 to 287, 2000” and Jpn. Pat. Appln. KOKAI Publication No. 2000-306859.
In the phase control ELA, an inverse peak type light intensity distribution (light intensity distribution in which a light intensity rapidly increases as a distance from a position having a minimum light intensity increases) is generated by the phase shift mask. The polycrystalline or amorphous semiconductor film is irradiated with light beams which periodically have the inverse peak type light intensity distribution. As a result, a molten region is generated in accordance with the light intensity distribution, and a crystal nucleus is formed in a portion which is disposed opposite to a position having a minimum light intensity and which is not molten or which first coagulates. When a crystal grows from the crystal nucleus toward periphery in a lateral direction (lateral growth), crystal grains having a large grain diameter (mono-crystal) are generated.
For example, when a liquid crystal display is manufactured, a ratio of a transistor forming region requiring the above-described crystallization in each pixel region is usually very small. In a conventional art, for example, the phase shift mask including a plurality of two-dimensionally arranged phase shift portions is uniformly irradiated with the laser beam. Therefore, a large part of the laser beam supplied from an optical illumination system does not contribute to the crystallization of the transistor forming region, and a so-called light amount loss is very large.
Moreover, as described above, in the conventional art, the semiconductor film is irradiated with light beams which have the inverse peak type light intensity distribution. In the light intensity distribution, the crystal nucleus is formed in the portion disposed opposite to the position in which the light intensity is minimized. Therefore, it is possible to control the forming position of the crystal nucleus. However, it is impossible to control the light intensity distribution in an intermediate portion between two inverse peak portions disposed opposite to each other.
In actual, in the conventional art, in general, the light intensity distribution in the intermediate portion involves irregular surges (wave-shaped distribution in which increase and decrease of the light intensity are repeated). In this case, in a process of crystallization, the lateral growth started toward the periphery from the crystal nucleus stops in a portion in which the light intensity decreases in the intermediate portion, and there is a problem that the growth of large crystals is inhibited. Moreover, even if a substantially uniform light intensity distribution is obtained in the intermediate portion, the lateral growth stops in an arbitrary position in this uniform light intensity distribution, and there is a problem that the growth of large crystals is inhibited.